The vitamin A metabolite, retinoic acid, has long been recognized to induce a broad spectrum of biological effects. For example, retinoic acid-containing products, such as Retin-A® and Accutane®, have found utility as therapeutic agents for the treatment of various pathological conditions. In addition, a variety of structural analogues of retinoic acid (i.e., retinoids), have been synthesized that also have been found to be bioactive. Many of these synthetic retinoids have been found to mimic many of the pharmacological actions of retinoic acid, and thus have therapeutic potential for the treatment of numerous disease states.
Medical professionals have become very interested in the therapeutic applications of retinoids. Among their uses approved by the FDA is the treatment of severe forms of acne and psoriasis. A large body of evidence also exists that these compounds can be used to arrest and, to an extent, reverse the effects of skin damage arising from prolonged exposure to the sun. Other evidence exists that these compounds have clear effects on cellular proliferation, differentiation and programmed cell death (apoptosis), and thus, may be useful in the treatment and prevention of a variety of cancerous and pre-cancerous conditions, such as acute promyleocytic leukemia (APL), epithelial cancers, squamous cell carcinomas, including cervical and skin cancers and renal cell carcinoma. Furthermore, retinoids may have beneficial activity in treating and preventing diseases of the eye, cardiovascular disease and other skin disorders. Major insight into the molecular mechanism of retinoic acid signal transduction was gained in 1988, when a member of the steroid/thyroid hormone intracellular receptor superfamily was shown to transduce a retinoic acid signal. Giguere et al., Nature, 330:624-29 (1987); Petkovich et al., Nature, 330: 444-50 (1987); for review, see Evans, Science, 240:889-95 (1988). It is now known that retinoids modulate the activity of two distinct intracellular receptor subfamilies; the Retinoic Acid Receptors (RARs) and the Retinoid X Receptors (RXRs), including their subtypes, RARα, β, γ and RXRα, β, γ. Different retinoid compounds exhibit different activities with the retinoid reactor subtypes. For example, all-trans-retinoic acid (ATRA) is an endogenous low-molecular-weight ligand which specifically modulates the transcriptional activity of the RARs, while 9-cis retinoic acid (9-cis) is the endogenous ligand for the RXRs, and activates both the RARs and RXRs. Heyman et al., Cell, 68:397-406 (1992); Levin et al., Nature, 355:359-61 (1992).
Although both the RARs and RXRs respond to ATRA in vivo due to the in vivo conversion of some of the ATRA to 9-cis, the receptors differ in several important aspects. First, the RARs and RXRs are significantly divergent in primary structure (e.g., the ligand binding domains of RARα and RXRα have only approximately 30% amino acid identity). These structural differences are reflected in the different relative degrees of responsiveness of RARs and RXRs to various vitamin A metabolites and synthetic retinoids. In addition, distinctly different patterns of tissue distribution are seen for RARs and RXRs. For example, RXRα mRNA is expressed at high levels in the visceral tissues, e.g., liver, kidney, lung, muscle and intestine, while RARα mRNA is not. Finally, the RARs and RXRs have different target gene specificity. In this regard, RARs and RXRs regulate transcription by binding to response elements in target genes that generally consist of two direct repeat half-sites of the consensus sequence AGGTCA. RAR:RXR heterodimers activate transcription by binding to direct repeats spaced by five base pairs (a DR5) or by two base pairs (a DR2). However, RXR:RXR homodimers bind to a direct repeat with a spacing of one nucleotide (a DR1). See Mangelsdorf et al., “The Retinoid Receptors” in The Retinoids: Biology, Chemistry and Medicine, M. B. Sporn, A. B. Roberts and D. S. Goodman, Eds., Raven Press, New York, N.Y., 2nd ed. (1994). For example, response elements have been identified in the cellular retinal binding protein type II (CRBPII), which consists of a DR1, and Apolipoprotein AI genes which confer responsiveness to RXR, but not RAR. Further, RAR has also been recently shown to repress RXR-mediated activation through the CRBPII RXR response element (Mangelsdorf et al., Cell, 66:555-61 (1991)). Also, RAR specific target genes have recently been identified, including target genes specific for RARβ (e.g., βRE), which consists of a DR5. These data indicate that the two retinoic acid responsive pathways are not simply redundant, but instead manifest a complex interplay and control distinct biological processes. For example, it has been demonstrated in leukemic cells, activation of RAR pathways regulates cell proliferation and differentiation, whereas activation of RXR pathways leads to the induction of apoptosis.
Retinoid compounds which are RAR and RXR modulators, including both RAR specific and RXR specific modulators, have been previously described. See, e.g., U.S. Pat. Nos. 4,193,931, 4,801,733, 4,831,052, 4,833,240, 4,874,747, 4,877,805, 4,879,284, 4,888,342, 4,889,847, 4,898,864, 4,925,979, 5,004,730, 5,124,473, 5,198,567, 5,391,569, 5,455,265, 5,466,861, 5,552,271, 5,801,253, 5,824,484, 5,837,725 and Re 33,533, and U.S. application Ser. Nos. 08/029,801, 872,707, 944,783, 08/003,223, 08/027,747 and 08/052,050; 60/004,897, 60/007,884, 60/018,318, 60/021,839. See also, WO93/03944, WO93/10094, WO94/20093, WO95/0436, WO97/12853, EP 0718285, Kagechika et al., J. Med. Chem., 32:834 (1989); Kagechika et al., J. Med. Chem., 32:1098 (1989); Kagechika et al., J. Med. Chem., 32:2292 (1989); Boehm et al., J. Med. Chem., 37:2930 (1994); Boehm et al., J. Med. Chem., 38:3146 (1995); Allegretto et al., J. of Biol. Chem., 270:23906 (1995); Bissonnette et al., Mol. & Cellular Bio., 15:5576 (1995); Beard et al., J. Med. Chem., 38:2820 (1995); Dawson et al., J. Med. Chem., 32:1504 (1989).
Breast cancer, like other malignant disease states, is characterized by a loss of cellular growth control followed by invasion of malignant cells into surrounding tissue stroma ultimately leading to metastatic spread of the disease to distant sites within the body. In 1987 over 180,000 new cases of breast cancer were diagnosed in the United States and there were 44,000 deaths due to breast cancer. Breast cancer is currently the second leading cause of cancer deaths in women and the leading cause of cancer deaths in women between the ages of 40 and 55. Population analysis on the incidence of breast cancer demonstrates that one-in-eight women in the United States will develop breast cancer at some point during their life. The primary therapy for breast cancer is surgery, either a partial or modified radical mastectomy with or without radiotherapy. This is typically followed by some form of adjuvant therapy.
The type of adjuvant therapy utilized is often dependant upon the estrogen receptor status of the tumor. Analysis of the hormone status of breast cancers demonstrates that 75% of all breast tumors are estrogen receptor positive and the majority of estrogen receptor positive tumors are found in postmenopausal women.
The anti-estrogen, tamoxifen, is presently the most commonly used drug worldwide for the treatment of breast cancer and approximately 66% of estrogen receptor positive breast cancers will respond to tamoxifen treatment. Tamoxifen is currently the first-line treatment for postmenopausal, estrogen receptor positive women with advanced breast cancer. The mechanism of action of tamoxifen in estrogen receptor positive breast cancer is thought to be due to competitive antagonism at the estrogen receptor of the estrogen driven growth of the tumor. Hence tamoxifen is a cytostatic, not a cytotoxic, agent.
It has previously been shown that as a chemopreventive, the RXR-selective retinoid LGD1069 (Targretin®) is as effective as the anti-estrogen tamoxifen (TAM) at inhibiting mammary carcinoma development in the NMU-treated rat. Gottardis et al., Can. Res., 56:5566-70 (1996).
Clinical evaluation of the efficacy of tamoxifen shows that a significant proportion of patients who initially respond to tamoxifen therapy will acquire resistance, and some on adjuvant tamoxifen therapy will suffer relapses. All advanced breast cancer patients eventually tend to develop tamoxifen resistance. The actual mechanisms underlying the development of tamoxifen resistance are most likely many fold and may involve decreased intra-tumor drug concentration, development of tumor cell clones that are now stimulated to grow in the presence of tamoxifen, and the development of estrogen receptor mutants among others.
Once a tumor develops tamoxifen resistance it will begin to proliferate even in the continued presence of tamoxifen. For breast cancer patients who develop tamoxifen resistance, secondary therapies include second-line hormonal agents such as progestins, aromatase inhibitors and LHRH agonists or cytotoxic chemotherapeutic agents. These commonly utilized second-line agents are at best only effective in approximately 25% of advanced cases. Hence, acquired tamoxifen resistance is the major cause of treatment failure in all stages of breast cancer. Accordingly, a need exists for improved methods and pharmaceutical compositions for treating anti-estrogen or tamoxifen resistant breast cancers.